This invention relates to a method and device for enhancement of digital signal transmission. More particularly, this invention relates to a method of converting digital signals to analog signals having a specified angular velocity with respect to rate of change of voltage and current, and a specified polarity whereby the limits on length and data transfer rates for metallic transmission lines, such as unshielded twisted pair, may be substantially increased.
Digital telecommunication systems such as personal computer networks utilize signal transmission media which must meet rigorous requirements in terms of electromagnetic emissions (e.g.; FCC Part 15) and transmission capability (e.g., crosstalk, attneuation VSWR, etc. . . .) for a wide range of signals and end-to-end distances. Presently, transmission means such as coaxial cable, twinaxial cable, shielded twisted pair and fiber optic cable are typically used to digitally interconnect electronic components in the network. Unshielded twisted pair, which offers advantages over other cable types in terms of cost, size of installed base, and ease of installation/maintenance, is utilized presently with severe limitations on distance and/or data transfer rates.
Presently used alternatives to the above noted transmission enhancement method that allow the use of unshielded twisted pair wiring with systems designed to operate with unbalanced or shielded media include: balun circuitry to enhance electromagnetic compatibility between cable types with dissimilar characteristic impedance levels and to isolate voltage reference levels between balanced and unbalanced media thereby eliminating ground loops; amplification circuitry to overcome limitations in dynamic range that may lead to unacceptable signal loss at the receiving end; filtration circuitry at the transmitting end to minimize electromagnetic emissions, and/or at the receiving end to reduce noise from external sources.
While these alternatives, used together or independently, offer economical solutions for providing signal transmission between telecommunication devices with unshielded twisted pair wire, they also suffer from several serious drawbacks that are overcome by this invention. For example, the use of baluns may allow signals, that radiate at an acceptable level over shielded media, to radiate beyond acceptable limits over unshielded twisted pair. Furthermore, the insertion loss and bandwith limitations of balun circuitry place restrictions on distance and data rates that are unacceptable for a broad range of applications. Amplification may overcome distance limitations to some extent on unshielded twisted pair data lines but is prone to generating increased electromagnetic emissions and, by itself, degrades signal to noise ratio. Filtering at the transmitting end to reduce out of band transmission is similar to the present invention in that high frequency Fourier components are attenuated, but is limited in that the filter bandwidth must be broad enough to pass digital pulses of varying duration. The design of such filters often requires that higher frequencies be transmitted with more power to overcome the greater noise and insertion loss associated with the wide bandwidth required by the system. Likewise, filtering circuitry at the receiving end must be of sufficient bandwidth to allow a wide range of pulse widths through to receiving circuitry without distortion or attenuation. Therefore, their ability to filter noise throughout that range is severely limited. These limitations are overcome by the present invention, which makes use of an analog signal of limited spectral content that permits transmission over unshielded twisted pair wire at very low power levels through matched impedance circuitry that is tuned to a specific angular velocity.
Economics suggest that, when active circuits, such as amplifiers, repeaters and other devices of the type described herein are used for signal transmission, the power required by remote circuitry be derived from the signal line in a "phantom power" approach. In prior art devices, phantom power is typically provided by an applied voltage on the secondary side of a signal output transformer. These methods include: the "green wire ground method" which imposes an AC or DC voltage on the line referenced from separate ground connections at both ends; various combinations of DC blocking capacitor methods which provide a DC voltage between separate signal carriers; and methods which tie AC or DC power to center tapped transformer elements connected directly to separate wire pairs. The prior art methods of transmitting power over signal carriers which employ a direct means of connecting power to the signal line are limited by the fact that output impedance of the power supply circuitry is not isolated from the signal, which is preferably controlled by the passive transformer elements alone. The present invention circumvents this problem by electromagnetically coupling AC power with the signal carriers. An inherent benefit of this design is the elimination of gound loops which give rise to electromagnetic interference in DC based phantom-power circuits.